The concept of a vector shift, or voltage vector shift, in an AC power system generally refers to a situation where a sudden change in a cycle length of a voltage wave of the AC power system occurs as a result of a sudden change in a load or power generation in the AC power system, for example. As a result, a single cycle becomes shifted with time, i.e., the cycle in question becomes longer or shorter. Consequently, the phase angle advancement momentarily does not correspond to a normal advancement rate related to a true frequency of the AC power system.
The detection of a vector shift in connection with an AC power system may be used in a vector shift relay used for detecting a disconnection of a generator from the network (an islanding situation), for example.
Another possible purpose for the detection of a vector shift in an AC power system is in connection with frequency measurement and frequency protection. Frequency measurement for frequency protection may in practice measure the advancement of the phase angle. In frequency protection the true network frequency is relevant and not an apparent frequency caused by disturbances. As already explained above, when connecting or disconnecting a load or power generation in the AC power system, for example, a vector shift can occur resulting in a situation where the phase angle advancement momentarily does not correspond to the normal advancement rate related to the true network frequency. The frequency measurement may erroneously see this as a frequency deviation and the frequency protection may unnecessarily trip. Thus, it would be beneficial to detect vector shifts in the system in order to be able to conclude that the result of the frequency measurement may not be correct during the vector shift phenomenon.
A problem with existing solutions for detecting a vector shift in an AC power system is that their sensitivity may not be sufficient and thus they may be able to detect only relatively large vector shifts.